For several years, we are witnessing in the automotive sector an increasing number and a generalization of the embedded electronic assistance systems designed to control the dynamic behavior of the vehicles, for safety purposes, such as path control (ESP) or braking (ABS) systems.
However, the correct operation of such control systems generally requires an accurate knowledge, in real time, of the orientation of the steered wheels of the vehicle.
For this purpose, it is known to measure, by using a suitable sensor, the instantaneous angular position of the shaft of the steering assist motor, then to deduce therefrom, knowing the gear reduction ratio of the reducer which connects the motor to the steering column, the angular position of the steering wheel, this angular position of the steering wheel being considered as representative of the position of the steering rack, and thus of the steering angle of the wheels.
However, the inventors have found that such evaluation method through calculation of the position of the steering members, and more particularly of the steering wheel, may be relatively inaccurate, according to the different life situations of the vehicle, the error of estimate (reported in equivalent rotation amplitude of the steering wheel) may reach in practice more than a dozen degrees.
By way of example, the inventors have in particular conducted the following experiment: they used, at the steering wheel of a steering mechanism, an angular position sensor whose accuracy was 1 (one) degree. Said steering wheel was driving a rack through a pinion, according to a pinion/rack connection which had a theoretical transmission ratio of 50 mm/revolution (the rack being supposed to be displaced linearly in translation by 50 mm when the pinion performs a full revolution). In view of this transmission ratio, and by considering the accuracy of the angular position sensor of the steering wheel, the absolute error of estimate of the linear position of the rack should have been in principle in the order of: 1 deg/(360 deg/revolution)*50 mm/revolution=0.14 mm.
However, the inventors have found that, in situations of intense steering, that is to say when a significant force was applied on the steering wheel (for example to simulate a clearing maneuver or a maneuver of leaving a parking position), said error of estimate of position of the rack, that is to say the deviation between the actual position (in translation) of the rack and the theoretical position of said rack calculated from the measured position of the steering wheel, could reach more than 1 mm, namely a value nearly seven times greater than the theoretically expected error.
In practice, on a new steering whose steering wheel is subjected to a high steering torque, in the order of 10 N.m, such error amounts to an angular deviation by ten degrees between the setpoint position given by the steering wheel and the effective position of the rack (expressed in the form of an angular position within the angular reference frame of the steering wheel).
It is then understood that such uncertainty does not enable guaranteeing optimal reliability and efficiency of the aforementioned path control or braking systems.
Furthermore, if possible, in absolute terms, to increase the accuracy of the measurement of the angular position of a member by the use of a dedicated sensor, located at the member whose position is desired to be known with accuracy, such solution represents in practice a significant additional cost, as well as an increase in mass and volume of the steering system, which runs counter to the intentions of lightening and reduction of costs constantly pursued by car manufacturers.